Large Delta T Thermal Cycling Induced Stress Accelerates Equilibrium and Transformation in Super DSS

Ping-Jui Yu,Shing-Hoa Wang,Shih-Che Chen,Hung-Wei Yen,Jer-Ren Yang,Tzy-Rong Lin,Horng-Yi Chang,Po-Kai Chiu

doi:10.3390/cryst10110962

Abstract

Based on the predicted phase diagram of super duplex stainless steel (DSS) calculated by Thermo-Calc, the maximum peak temperature 1100 °C was selected to ensure no σ phase existence. This target temperature fell into the two-phase solid solution (SS) region. A series of different thermal cycling tests were carried out with the notations of 2SS, 2SS + 3 cycles, 2SS + 7 cycles, 2SS + 13 cycles, and 2SS + 20 cycles. It was found that the trend of two-phase volume ratio variation by thermal cycling followed the predicted thermodynamic equilibrium trend. After 2SS + 7 cycles, the ratio of two-phase δ/γ tended toward the ideal 1:1. According to the electron backscatter diffraction (EBSD) analysis, the δ phase crystal orientation changed from the most frequent directions of <001> and <111> of the as-received sample to the most frequent orientation of <113> after two SS treatments. While the γ phase grain always remained at <101> orientation. The grain boundary misorientation angles of the γ grains were relatively stable, ranging from 53° to 63°, but those of the δ grains were widely distributed actively presuming the lattice rotation. The Kernel Average Misorientation (KAM) value of the local strain in face center cubic (fcc) γ grains was varied and greater than that of the body center cubic (bcc) δ phase, indicating that the former, with a large grain boundary misorientation had larger local deformation than the latter, which possesses wide random misorientation angle distribution.

Highlights

Duplex stainless steel (DSS) is a two-phase stainless steel comprising equal volumes of the δ phase and γ phase
After local plastic deformation from thermal cycling, two dislocation configurations variationsobserved of different phase grain orientations and[51], Kernel Average Misorientation (KAM)
Boundary misorientations wereThe theoretically in the deformed microstructure i.e.,grain (i) statically stored dislocations were investigated after different cyclic numbers of thermal processes with large

Summary

Introduction

Duplex stainless steel (DSS) is a two-phase stainless steel comprising equal volumes of the δ phase and γ phase. The ferrite raises the corrosion resistance mainly against pitting, stress, and crevice corrosion. It has high strength and superior corrosion resistance due to the presence of alloying chromium (Cr), molybdenum (Mo) and nickel (Ni). SAF 2507 (S32750) is developed mainly for chloride-containing environments, like seawater. It contains higher amounts of Cr, Mo and N, which improve the microstructural stability [1,2,3]. The high Cr alloying can increase the corrosion resistance and the Ni improves the toughness. The N can increase the strength and resistance against pitting corrosion [3]. The optimization of the corrosion resistance and the mechanical properties of DSS rely on the precise

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Discussion

Conclusion